The human brain is an expensive tool: A huge proportion of human genes are thought to be involved in constructing, wiring up, and maintaining the nervous system. Neuroscientists hope the completed genome will help them to nail down the brain's share. Current estimates range from “a fair chunk” of the genome to “40%” to “most.”

No one knows what all these genes do, but placing them on gene chips to see which ones are expressed by developing neurons is like “having a new type of microscope, a new way of looking at cells,” says neurobiologist Ben Barres of Stanford University. His team is using such chips, as well as protein analysis, to spot molecular signals passed between neurons and support cells called glia early in development, when neurons start transmitting messages.

The completed genome will also accelerate the search for genes at fault in neurodegenerative diseases. Neurogeneticist Huda Zoghbi of Baylor College of Medicine in Houston looks for candidate genes in the Drosophila genome, then tries to find homologs in the human sequence. Making the jump from fruit fly to human used to take a year of lab time, she says; now she'll be able to search computerized databases to find candidate genes in minutes.

Other neuroscientists hope the genome will help solve otherwise intractable questions about human behavior. For example, psychiatrist Eric Nestler of the University of Texas Southwestern Medical Center in Dallas and computational biologist David Landsman of the National Library of Medicine in Bethesda, Maryland, point out in this week's issue of Nature that newly identified genes might help make sense of addiction. Cocaine acts on certain dopamine transporters, which differ between people; correlating people's transporter subtypes with their propensity for cocaine addiction might reveal why some people are more vulnerable to the drug than others, they suggest.